Re: ROMs replacement

On 9/16/2012 8:04 AM, Gerrit Heitsch wrote:
> On 09/16/2012 01:31 PM, silverdr@wfmh.org.pl wrote:
>>
>> On 2012-09-16, at 12:57, Gerrit Heitsch wrote:
>>
>>> The simplest way to achieve that is a 74LS257.
>>
>> Yes, or 157.. as we don't need the cutoff of the output anyway.
>
> Yes, but the advantage of the 74LS257 is that if you do hardware work 
> on the C64, C16, +4... you will have some 74LS257 at home.
>
> The LS157 doesn't have tristate outputs, but still has an enable 
> input, so no matter how you slice it, it's a signal you need to take 
> care of.
>
>
>> Yup, levels are very much my worry here but I wanted to avoid adding 
>> another IC, especially one with most pins unused.
>
> After thinking about it for a bit... You can also use a 74LS00 (or 
> 74HCT00) which is fully used up in the process and has 2 pins less:
>
> How to set up:
>
> Input1 is A12, it gets fed into one input of one NAND.
>
> Input2 is your switch. It gets fed into one input of another NAND. 
> Remember the pullup here.
>
> The select signal (_Char-ROM) is directly fed into the other input of 
> the NAND with A12. It is also inverted by using one NAND (second input 
> tied HIGH) and then fed into the second input of the NAND that takes 
> your switch input.
>
> The outputs of the 2 NANDs are then fed into the last available NAND 
> and its output is A12 for the EPROM.
>
> Should look like this:
>
> http://ecee.colorado.edu/~ecen3100/lab2_files/2-1_NAND_mux.PNG
>
> Gerrit
>
>
>
>       Message was sent through the cbm-hackers mailing list
Looking at the original schematic, I sketched one up using a '148 and a 
'257.  With the '257 as Gerrit suggested (the NAND idea would also work) 
and the 148 handling the 3 signals.  The '148 takes care of OE/CE with 
it's GS pin (14), and the design requires just 2 ICs and 1 resistor (to 
hold the CHARROM switch position in high unless shorted through the 
switch/jumper.

Jim


-- 
Jim Brain
brain@jbrain.com
www.jbrain.com


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